HEREDITY AND VARIATION 



in the natural habitat. This normal course of inheritance can be observed 

 and interpreted, as has been explained. From it we learn the laws of the 

 transmission of the hereditary units; we learn the mechanism of heredity. 

 How does a particular complement of genes, or genome, influence the course 

 of development of a zygote in such a way as to condition the appearance of 

 the phenotype? What is the relation of gene to character? In this, as in 

 many fields of study, a knowledge of what happens under abnormal or un- 

 usual conditions may clarify our understanding of the normal situation. In 

 the examples cited to illustrate Mendelian principles, a certain combination 

 of genes always gives a certain type of individual. However, if the genetic 

 combination remains the same but the environment is changed, the in- 

 dividuals may be different (p. 167). For example, the red primrose has red 

 flowers if kept at a temperature ranging from 15° to 20°C. A plant with the 

 same genes, reared at a temperature of 30° to 35°C. with other environ- 

 mental conditions unchanged, produces white flowers. If a plant with white 

 flowers is brought into a room at 15° to 20°C., the flowers that develop later 

 will be red. The effect of the genes for color of flower is limited by the 

 temperature of the environment in which the flowers develop. That the gene 

 is not altered is shown by the ability of the plant to cease producing white 

 flowers and give rise to red ones when kept at a different temperature (cf. Fig. 

 6.21). The same type of effect has been demonstrated in Drosophila. 

 A certain race of the fruit fly is distinguished from the normal by the fact 

 that there are very few black bands on the abdomen. When this race is 







Fig. 6.20. Chromosomal aberrations seen in the paired sahvary-gland 

 ciiromosomcs of Drosophila. A, part of the pair of X-chromosomes, 

 one of which has lost an internal section by deletion (cf. Fig. 6.23); 

 the bent portion of the normal partner indicates the length of the deleted 

 section. B, the end of a pair of X-chromosomes, one of which has lost a 

 terminal section. (,4, from T. S. Painter, 1934, Journal of Heredity, vol. 

 25; B, from M. Demerec and Margaret E. Hoover, 1936, Journal of 

 Heredity, vo\. 27.) 



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